Hydrocracking is a well established process in the petroleum refining industry and is conventionally employed either for fuels (distillate) production or for producing lubricants from poor or marginal quality crude sources. In both types of hydrocracking operation, the feed, usually a high boiling material such as a vacuum gas oil (VGO) or a catalytically cracked fraction such as light cycle oil (LCO) or heavy cycle oil (HCO) is contacted with a bifunctional catalyst in the presence of hydrogen under elevated temperatures and pressures. The catalyst has an acidic function which is typically provided by an amorphous support such as alumina or silica-alumina or by a large pore size crystalline zeolite such as zeolite X or zeolite Y. Large pore size materials of this type are used in hydrocracking operations in order that the bulky, polycyclic aromatic components in the typical feedstocks may obtain access into the internal pore structure of the catalyst where the characteristic hydrocracking reactions mostly take place. The metal function is provided in order to promote the hydrogenation reactions which occur and is usually provided by means of a base metal of Group VIA or VIIIA of the Periodic Table (IUPAC Table) such as nickel, cobalt, tungsten or molybdenum or, in some cases, by a noble metal such as platinum or palladium. Combinations of base metals from both Groups VIA and VIIIA are preferred, for example, nickel-tungsten, cobalt-molybdenum have been found to provide superior results. Other metals may be present to promote or suppress certain hydrogenative type functions, for example, tin may be present as described in U.S. Pat. No. 3,535,227. Temperatures in hydrocracking are typically at least about 500.degree. F. (about 260.degree. C.) and are typically in the range of 650.degree.-850.degree. F. (about 345-455.degree. C.) with higher temperatures being disfavored for thermodynamic reasons. Hydrogen pressures are typically at least about 400 psig (about 2860 kPa abs.) and depending upon the mode of operation are frequently in the range of about 1000 to 2500 psig (about 7000 to 17,340 kPa abs.). Hydrogen circulation rates are typically at least about 1000 to 5000 SCF/Bbl (about 180 to 890 n.l.l.). Space velocities are typically about 0.5 to 5, more commonly 0.5 to 2 LHSV (hr..sup.-1).
As described above, hydrocracking conventionally employs a support material having acidic functionality such as amorphous alumina, silica-alumina or a crystalline support such as the large pore size zeolites e.g. zeolite X or zeolite Y. Recently, the use of zeolite beta for this purpose has been proposed and it has been found to possess a number of significant advantages for the hydrocracking process. It has been shown that zeolite beta, unlike typical large pore size hydrocracking catalyst supports such as zeolite X and zeolite Y, is paraffin-selective in the presence of aromatics and tends to convert paraffins in the feed in preference to the aromatics with a preferential mode of attack towards the relatively high boiling paraffins. Moreover, zeolite beta possesses a marked propensity to isomerize the higher molecular weight paraffins so that not only is it able to effect a bulk conversion of the feed by cracking to lower boiling range products but, in addition, is able to isomerize waxy paraffins which are present in the feed to less waxy isoparaffins so that a marked reduction of product pour point is noted, especially in the bottoms fraction (usually 650.degree. F.+, 345.degree. C.+). By contrast, conventional zeolite and amorphous hydrocracking catalysts are generally aromatic selective so that aromatics in the feed are converted in preference to paraffins. This behavior is illustrated in European Patent Application Publication No. 94827 which compares the results of hydrocracking using a zeolite beta based catalyst and a catalyst based on zeolite Y. The use of the conventional zeolite Y based catalyst results in a significant net increase in the proportion of paraffins in the high boiling fraction with a major reduction in the proportion of aromatics. By contrast, the zeolite beta based catalyst effects a significantly smaller increase in paraffins and a smaller decrease in aromatic content. At the same time, the pour point of the high boiling fraction is significantly reduced by the use of zeolite beta while remaining essentially unchanged with the conventional hydrocracking catalyst (see Examples 3 and 4 of EP No. 94827).
Other hydroprocessing applications exploiting the properties of zeolite beta have been described in the patent literature. See, for example, U.S. Pat. Nos. 4,554,065, 4,568,655, 4,501,926, 4,518,485 and EP No. 163,449.
In conventional hydrocracking operations, a hydrotreater usually preceeds the hydrocracking reactor in order to effect a preliminary saturation of aromatics present in the feed and, in addition, to convert organic nitrogen and sulfur to inorganic forms (ammonia, hydrogen sulfide) which may optionally be removed in an interstage separation prior to entering the hydrocracking reactor or reactors in which the conversion to lower boiling products takes place. In a typical fuels hydrocracking operation where the objective is to maximize naphtha and distillate production, the unconverted fraction may be recycled to extinction, either in a second stage reactor or by recycle to the first stage hydrocracking reactor. Operation with two hydrocracking reactors is frequently preferred in order to extend catalyst cycle life, with the total conversion being split between the two hydrocracking reactors. In a lube hydrocracking operation, the high boiling lube product is preserved although lower boiling materials including naphtha and distillate are removed for separate disposition. In this case, however, the objective of the process is to effect a conversion of aromatics to materials which are relatively more paraffinic and which have better properties as lubricants. This is effected both by a process of ring saturation to naphthenic type materials as well as to saturation followed by ring opening reactions to form paraffinic chains which may, however, remain attached to aromatic moieties.
As described above, fuels hydrocracking generally operates with extinction recycle to convert the feed to lower boiling materials, typically to 650.degree. F.- (about 345.degree. C.-) products, usually with major amounts of naphtha. To do this, the recycle conventionally enters between the hydrotreater and the hydrocracker, either upstream or downstream of the interstage separator.